1. Field of the Invention
The present invention relates to a glass antenna for an automobile, and in particular, defogging heater wires on a rear window glass of the automobile are used as antenna elements.
2. Description of the Prior Art
The inventors proposed in Japanese patent application laid open No. 3-49402 (1991) a glass antenna for an automobile using defogging heater wires on a rear window of the automobile as antenna elements. FIG. 1 shows this type of the glass antenna for an automobile. In FIG. 1, a plurality of heaters or wires 3 is printed on the defogging area 2 of the rear glass 1, while an FM antenna 4 is printed above the upper portion of the uppermost heater 26.
Each heater 3 has an end connected to a power bus 5 or 6 and another end connected to a relay bus 7. The power bus 5 is connected to a power source +B having for example 12 volts through a power line 8 and a choke coil 9. The power bus 6 is connected to ground through another power line 10, another choke coil 11 and a switch 12. The choke coils 9 and 11 have high impedance characteristics against an AM frequency band to be received and are wound several turns around the toroidal core for a high frequency use. A decoupling capacitor 25 is coupled between the main power source +B and ground to reduce a power source noise.
When the heaters 3 are used for defogging the rear glass 1, the switch 12 is turned on to connect the choke coil 11 to ground. Twelve volts is then supplied through the choke coils 9 and 11 and the power lines 8 and 10 respectively to heat the upper group of the heaters 3 between the power bus 5 and the relay bus 7 and the lower group of the heaters 3 between the relay bus 7 and the power bus 6. When the heaters 3 are used as the AM antenna, an AM signal is picked up through the power line 8 or 10.
FIG. 1 also shows an impedance matching circuit 13 connected to an FM antenna provided on the rear glass 1 as proposed in Japanese utility model application laid open No. 2-64219 (1990), entitled The impedance matching circuit 13 comprises a reactance circuit consisting of a coil and a variable capacitance or varactor diode so as to perform an impedance matching for a radio receiver viewed from the FM antenna through a cable 27 at a random frequency by using a frequency selective signal applied from the radio receiver through the cable 27.
The sensitivity of the FM antenna 4 as shown in FIG. 1 has a bidirectional characteristic. Particularly, the direction perpendicular to that of the maximum sensitivity has a poor sensitivity. The FM antenna 4 provided along the rear window is not suitable for a vehicle antenna since a direction of electric wave with respect to the antenna is often changed over 360 degrees while driving the automobile.
It is therefore necessary to construct a diversity antenna system by using FM main and subsidiary antennas to complement their directivities. The FM main antenna comprises the FM antenna 4 while the FM subsidiary antenna comprises the AM antenna in the defogging area 2 as shown in FIG. 1. However, the resistance (real) and reactance (imaginary) components of the antenna impedance at the feeding point adjacent to the upper portion on the power bus 5 of FIG. 1 change within the FM frequency band as shown by curves A in FIGS. 2 and 3 respectively when the heaters 3 are used as the FM subsidiary antenna. They show parallel resonance characteristics each having a peak within the FM frequency band. When such an FM antenna with the peak impedance characteristic is employed with the conventional dynamic impedance matching circuit to match the impedance of the transmission cable 27, the matching circuit can not trace at respective frequency the non-monotonic impedance which non-monotonically change with respect to the frequency due to a saw-toothed voltage sweep upon a channel selection of a FM program. The mean gain value of the antenna system with the impedance matching circuit is lower than that of the antenna system without the impedance matching circuit over the FM frequency band. Since the choke coils 9 and 11 generally have an inductance of 600 to 1300 micro-henries and a stray capacitance of several to several ten picofarads, they have sufficiently high impedance to prevent the AM signal induced by the heaters from going through the body of the automobile in the AM band However, since they have low impedance in the FM band, they result in the FM signal going through the body. Therefore, since a sufficient antenna voltage in an open state can not be obtained, the gain of the antenna system is lowered.